The present invention relates to ink jet printers and, more particularly, to an ink supply system for supplying ink to the print head of such a printer.
An number of different types of ink supply systems have been utilized in the past for supplying ink under pressure to the print head of an ink jet printer. Typically, the print head of an ink jet printer defines a fluid reservoir to which ink is applied and at least one orifice from which a fluid filament emanates. Mechanical disturbances are applied to the fluid filaments, as for example by means of a piezoelectric transducer, to stimulate the filaments to break up into jet drop streams. As drops are formed from the fluid filaments, the drops are selectively charged and, thereafter, are deflected by an electrostatic field such that they are separated into print and catch trajectories. The drops in the print trajectories strike a print receiviing medium, such as a paper web, while the drops in the catch trajectories are directed to one or more drop catchers, which ingest the drops and return them to the fluid supply system for reuse. One such prior art printer is shown in U.S. Pat. No. 3,701,998, issued Oct. 31, 1972, to Mathis.
One type of fluid supply system which has been utilized with such ink jet printers is shown in U.S. Pat. No. 3,761,953, issued Sept. 25, 1973, to Helgeson. The Helgeson ink supply system includes a fluid recirculation path from a fluid pump to a pressure regulation tank and back to the pump. Fluid is withdrawn from the pressure regulation tank and supplied to the print head and the ink in the recirculation path is replenished from a supply tank. While providing fluid to the print head at a desired pressure, the flow rate of the ink is not controlled. Additionally, the supply system of Helgeson is relatively complicated and requires a substantial number of components.
A significant problem encountered with ink jet printers is the difficulty of providing a start up of the printer in which the jet drop streams are formed without wetting other print components. As the flow of ink through the print head orifices begins, the jet drop streams initially established tend to be somewhat unstable, both in trajectory and in drop size. This instability may also reappear at shut down of the printer as the fluid flow through orifices is terminated.
U.S. Pat. No. 4,042,937, issued Aug. 16, 1977, to Perry et al., discloses an ink supply system in which sequencing of purging, start up, print operation, and shut down of the printer are controlled by a pair of solenoid-actuated valves connected in the inlet and outlet lines of the print head. The inlet valve is connected between a pump and the print head, while the outlet valve is connected between the print head and the supply tank which provides ink to the pump. Start up is accomplished by filling the print head with ink, closing the inlet valve to permit pressure to build behind the inlet valve to a level significantly greater than that required for operation and, thereafter, opening the inlet valve. At shut down, the inlet valve is closed while the outlet valve is held open, thus creating a negative pressure in the head. The Perry et al supply system does not include a provision for controlling the fluid flow rate to the print head but, rather, simply operates with the pump providing whatever flow of ink to the head may result from the opening and closing sequences of the valves.
Another problem encountered with ink jet printers is that air may become trapped within the print head ink reservoir. Air pockets or bubbles in the print head may inhibit proper printer operation due to their compressibility. U.S. Pat. No. 3,974,508 issued Aug. 10, 1976, to Blumenthal, discloses an ink jet printer in which air bubbles are purged from the print head by passing the ink from an inlet line through the print head to an outlet line at a relatively high flow rate. This sweeps out air pockets that might otherwise remain in the print head.
An ink supply system designed for solving both problems is shown in U.S. Pat. No. 4,314,264, issued Feb. 2, 1982, to Bok et al. It has been found that within a range of subatmospheric static pressures within the print head, in particular, between approximately 2 and 11 inches of water below atmospheric pressure, that prior to start up, ink will not weep through the print head orifices nor will air be ingested thereinto. The Bok ink supply system includes a pump for providing ink under pressure from an ink supply tank to the pump outlet. Ink is then carried by a supply line to the print head, and may be returned therefrom to the ink supply tank through a return line. Additionally, a plurality of ink lines is provided for returning ink from the pump outlet to the pump inlet. Each of these lines includes a flow restriction, for providing a different fluid flow impedance within each line. A valve system is included for directing a portion of the ink from the pump outlet into a selected one of the ink return lines. By switching between the various return lines, static pressure within the print head may be brought in a stepwise fashion within the ideal start up range. A print head outlet valve may then be closed, creating a pressure shock wave which travels into and through the print head. The shock wave produces a virtual instantaneous pressure increase, sufficient to cause proper commencement of jet drop streams.
As can be readily seen, however, changes in fluid flow impedance within the ink supply system may be sufficient to alter the static pressure within the print head immediately prior to start such that it is not within the optimum pressure range. This is true not only for flow impedance changes within the return lines, but also within the entire recirculation system. Such changes may result, for example, from the deposit of dirt or particles of dried ink within the fluid system. Thus, it can be seen that adjustment of the system may be somewhat critical, if weeping or air ingestion is to be avoided.
Accordingly, it is seen that there is a need for a fluid supply system for an ink jet printer in which the supply fluid to the print head is controlled in a simple, reliable manner, and in which start up and shut down are reliably controlled.